ASCA: Frequently Asked Questions for the Non-Specialists
This page is primarily aimed at astronomers who normally do not work on X-ray data, and at graduate students and advanced undergraduates embarking on X-ray analysis.
See also the HEASARC Cookbook, which answers many similar questions for X-ray and gamma-ray astronomy in general.
- Q1. Is ASCA relevant to my field of study?
You may want to check the ASCA
Science Highlights pages to see what has been accomplished with
ASCA. This should give you a good idea of the type of science
that can be done with ASCA data.
- Q2. What's the difference between ASCA and Chandra (or XMM-Newton or ROSAT ...)?
ASCA is primarily a spectroscopy mission; you can refer to a
recent, current, and future missions.
In addition, observers of variable objects may be interested to note that ASCA obtained useful data between 1993 April and 2000 July, and that a typical observation lasted for a day or so, resulting in about 40,000 s of useful on-source data for each observation.
- Q3. Is there an ASCA observation of my favorite object?
As of 1998 April, there were 240 observations of stars,
58 of cataclysmic variables, 198 of X-ray binaries, 387 of
supernova remnants and diffuse Galactic emission, 152 of
normal galaxies, 485 of active galactic nuclei, 372 of
clusters of galaxies, and 240 "surveys and misc."
observations (see the
article for more on the status of archive as of 1998 April).
The ASCA archive has grown by about 30% since then.
You can search for individual objects in the ASCA archive using Browse.
- Q4. How do I know if a specific ASCA observation has been published or not?
Unfortunately, we do not have an easy way to check this yet.
You can search for papers that mention ASCA and
the name of your favorite object using
Or you can look through our ASCA
- Q5. How do I download ASCA data on my favorite object?
There is a good description of this in the
- Q6. Which files should I download?
Spectra, Lightcurves, and Sky Images
(or GIF Sky Images) are useful for quick-look purposes.
However, these have been generated automatically during processing,
and we cannot guarantee a sufficient accuracy for publication purposes.
You can perhaps start from Cleaned Events Data, and extract images, spectra, or light curves yourself. This will probably work fine for the majority of the cases. Alternatively, you can start from Unscreened data, and screen them yourself to achieve the maximum possible signal-to-noise.
You almost never need to go back to the raw telemetry or obtain calibration and auxiliary housekeeping files (a few essential auxiliary files, such as the filter file, will be included when you download cleaned or unscreened data via Browse).
For more details of all the files in the ASCA archive, consult the Getting Started Guide
- Q7. How were the "Cleaned Events Data" cleaned?
Here are the screening criteria used in processing.
These criteria have been developed
through experiences to exclude data dropouts and times of high background.
In some cases, small a small amount of high-background data may have
been left in, though this rarely affects the average spectrum of the
For more details (and explanations on how you can screen your own data), see the Screening chapter of the ASCA data reduction guide.
- Q8. What software should I use to analyze ASCA data?
We have developed, and continued to support,
XANADU suites of software
for X-ray data analysis in general, and ASCA data analysis
in particular (here is list
of ASCA specific FTOOLS).
It is also possible to use IDL to analyze ASCA data.
- Q9. What is the essential documentation for analyzing ASCA data?
The ASCA Data Reduction Guide
(also known as the ABC guide) is very comprehensive. We recommend
browsing through this guide to familiarize yourself with the basics.
Also, whenever you have a question, you should check the relevant
- Q10. Can I combine the data from different instruments on ASCA?
You cannot safely combine the event data files. However, once
you have extracted the data products, (images, spectra, or light
curves), you can combine them to some extent. In particular,
combining data from the 2 GIS detectors is reasonably safe,
while combining data from the 2 SIS detectors is somewhat
more complicated, this can be done without too much harm. You
need to pay attention to how backgrounds and instrument responses
need be combined, too.
- Q11. How can I "extract" images, spectra, and light curves from screened data files?
Read your screened data files into
Xselect and use the
extract command. By default, it will "extract"
the product(s) for the entire active area of the detector (except
those areas that have been screened away). To extract the spectrum
of a point source in the field, you need to create a region file
(most conveniently in SAOimage; see the
Filter chapter of the ABC guide)
and apply this in Xselect using filter region command.
For point sources, a circular extraction region of 2.5-3.5 arcmin (for SIS) or 6 arcmin radius (for GIS), centered on the source, work best. For both instruments (but particularly for SIS), though, make sure that your extraction region falls entirely within the active region of the detector.
Xselect allows other types of filtering that may be appropriate for your scientific aims: by time, by binary or spin phase, by event energy, and by source intensity. Consult the Filter chapter of the ABC guide and the filter command section of the Xselect guide for details.
- Q12. What about the background?
Because ASCA is imaging, the background is much lower than in
collimated instruments: For a typical point source extraction region,
it is of the order of 0.02 cts s-1, when normal screening
criteria have been applied to the data. Because the background is a
combination of cosmic X-ray background and instrumental (particle)
background, it is both variable in time and variable from one location
of the sky to another.
Depending on the brightness of the source, and the nature of your scientific purposes, you can either use blank sky background (from the identical section of the detector taken from observations of fields without any obvious sources) or local background (parts of the detector without obvious sources from your own observation). The latter works because, to first approximation, the background rate is constant across the SIS field-of-view and across the inner part of the GIS field-of-view.
For more on background subtraction, please see the relevant sections of the ABC guide (spread across Spectral Analysis, Temporal Analysis, and Spatial Analysis chapters).
- Q13. Where can I obtain the spectral response?
Spectral response comes in two parts, redistribution matrix file (RMF)
that describes the distribution of signals for a monochromatic input,
and ancillary response file (ARF) which describes the effective area curve.
For the GIS, you can obtain the RMF files from the calibration database. For the SIS, you need to generate one using the FTOOL sisrmg: this is because the response depends on the epoch of your observation, as well as various instrumental settings. Sisrmg takes your extracted spectral file as input (the second input, the ARF file, should be left as "none").
The ARFs are strongly dependent on your extraction region. For both GIS and SIS, you need to generate them using the FTOOL ascaarf.
For more details, please consult the Spectral Analysis chapter of the ABC guide.
This file was last modified on Wednesday, 03-Mar-2004 11:30:37 EST
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